If you are on 3.1.3 Classic, it is a bug with IC2 Reactor control, to fix it you need to delete it from the mods folder and install v.1.1.11 from here or just downgrade to 3.1.2 (which could damage your worlds, backup first as always)
Just my luck lol, come back from holiday, find out im white-listed, mini party whoop whoop aaannnnnd the servers gone. But not much longer? (100+ mods ftw!)
The login servers are down but minecraft.net is back up. Proof: http://xpaw.ru/mcstatus/ (My go-to when I can't log-in, checking the Mojang employee's Tweets also can giveaway that something is up)
Edit: Lol ^^